The present disclosure relates to an optical device for scanning a laser beam across a predetermined target, e.g. a photoconductive drum and an image forming apparatus with the same.
An optical device for scanning a laser beam across a predetermined target is used as an exposure device (laser scanner unit (LSU)) in an image forming apparatus such as a copier, a printer or a facsimile machine. The exposure device scans a laser beam across a photoconductive drum based on image information to form an electrostatic latent image on the photoconductive drum.
The exposure device includes, as basic constituent elements, a laser light source for outputting a laser beam, a collimator lens, a cylindrical lens, a polygon mirror, an fθ lens, a reflecting mirror and the like. Out of these, the fθ lens has an important function of scanning the laser beam reflected by the polygon mirror across the photoconductive drum. Accordingly, the fθ lens needs to be accurately mounted.
Since image forming apparatuses have been required to have high image quality and high image density in recent years, the fθ lens of the exposure device is mostly designed to have an aspherical surface. Although the aspherical fθ lens has an aspherical shape in a main scanning direction of the laser beam, it is difficult to recognize asymmetry on the external appearance when a degree of asymmetry is small. Thus, it is difficult for an operator to visually judge a correct orientation of the fθ lens. Therefore, there is a need for a means for preventing the inverted mounting of the fθ lens.
As a conventional technology for preventing the above inverted mounting, it is known to process the fθ lens. This fθ lens includes, on an output surface side, a pair of upper and lower first projecting portions and a second projecting portion at a predetermined distance in a longitudinal direction from the first projecting portion. On the other hand, a housing in which the fθ lens is mounted includes positioning pins for positioning the fθ lens by being engaged with the first projecting portions, and an interference pin. The interference pin interferes with the second projecting portion when the fθ lens is mounted in an inverted manner. This enables the operator to identify a correct orientation of the fθ lens by knowing whether or not there is any interference by the interference pin in mounting the fθ lens into the housing. In this way, the inverted mounting of the fθ lens is prevented.
However, more and more fθ lenses have been formed to have an aspherical surface in recent years, and it has been more difficult for an operator to visually judge a correct orientation of the fθ lens in mounting the fθ lens. Thus, it is hard to say that the inverted mounting of the fθ lens can be reliably prevented by the inverted mounting preventing means of the conventional technology, wherefore there is a need for a more accurate inverted mounting preventing means.
An object of the present disclosure is to provide an optical device capable of reliably preventing the inverted mounting of an optical element such as an fθ lens, an optical member and an image forming apparatus with the same.